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Postretirement Health Care and Life Insurance Benefits (Details 3) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Postretirement Health Care and Life Insurance Benefits [Member]      
Weighted-average assumptions used to determine benefit obligations      
Discount rate 4.50% 4.10%  
Measurement date Dec. 31, 2015 Dec. 31, 2014  
Weighted-average assumptions used to determine net periodic benefit cost      
Discount rate 4.10% 4.90% 4.21%
Measurement date December 31, 2014 December 31, 2013 December 31, 2012
Assumed health care cost trend rates, One percentage point increase      
One Percentage-Point Increase Effect on total service and interest cost components [1] $ 3.8    
One Percentage-Point Decrease Effect on total service and interest cost component [1] (3.4)    
One Percentage-Point Increase Effect on total postretirement benefit obligation [1] 71.3    
One Percentage-Point Decrease Effect on total postretirement benefit obligation [1] $ (62.3)    
Pre-Medicare [Member]      
Assumed health care cost trend rate      
Health care cost trend rate assumed for next year 6.60% 7.00%  
Rate to which the cost trend is assumed to decline (the ultimate trend rate) 4.75% 4.75%  
Year that the rate reaches the ultimate trend rate 2021 2021  
Post-Medicare [Member]      
Assumed health care cost trend rate      
Health care cost trend rate assumed for next year 5.80% 6.00%  
Rate to which the cost trend is assumed to decline (the ultimate trend rate) 4.75% 4.75%  
Year that the rate reaches the ultimate trend rate 2021 2021  
[1] In addition to the effect on total service and interest cost components of expense, changes in trend rates would also increase or decrease the actuarial gain or loss amortization expense component. The impact on actuarial gain or loss amortization would approximate the increase or decrease in the obligation divided by 10.49 years at January 1, 2016.